1. Field of the Invention
The present invention relates to a short-range wireless communication system, and more particularly to a short-range wireless communication system for processing handoffs as mobile terminals move and a handoff processing method therefor.
2. Description of the Prior Art
The short-range wireless communication system such as the Bluetooth, which performs communications by using the Time Division Duplex (TDD) scheme, is designed in order for an Access Point (AP) connected to the base network (wired/wireless) to serve as a master, and for each mobile terminal to serve as a slave. At this time, if a mobile terminal recognizing an AP as a master is moving into a different wireless range, it is necessary to automatically switch a current communication channel of the mobile terminal into a different communication channel for the different wireless range. Such communication channel switches according to the movements of a mobile terminal are referred to as handoffs, which can be stated as a very important function in the short-range wireless communication system.
The conventional short-range wireless communication system generally uses the hard handoff referred to as the break-before-make handoff that breaks an existing communication channel before a new communication channel is made, to thereby enable link re-establishments.
However, the hard handoff algorithm has a problem of taking a somewhat long time for the link re-establishments after a handoff occurs, so diverse methods for overcoming the problem have been proposed recently.
There are three typical approaches as follows for solving the time delay problem in re-establishing a link when a handoff occurs.
First, there is the technology of Access Point Roaming (APR) proposed by the Bluetooth Special Interest Group (Bluetooth SIG)(Reference material: Bluetooth SIG, PAN Access Point Roaming, Rev 0.51, June, 2002). APR is the technology in which neighboring Access Points (APs) share information on mobile terminals to re-establish a link via only page scan procedures without an inquiry scan. For the APR technology, there exists the Personal Area Network User (PANU) mode by which the handoff starts and the Network Access Point (NAP) mode by which the handoff starts, wherein the occasion that the handoff is triggered by the PANU is referred to as the requested handoff and the occasion that the handoff is triggered by the NAP is referred to as a forced handoff. In both of the modes above, the mobile terminal inquiry information is transferred to neighboring APs at the time that the mobile terminals are disconnected with each other and a link with only page scan procedures is re-established without an inquiry scan.
FIG. 1 is a view for showing signal flows when a handoff is triggered by a PANU in the APR. The handoff is triggered by a Blue Network Encapsulation Protocol (BNEP) handoff message from the PANU to the AP(NAP B)(refer to (1) of FIG. 1), and, if the AP(NAP B) is disconnected at the link layer (refer to (2)), the AP(NAP B) transfers in the wired manner an Inter NAP Communication (INC) message including the inquiry information of the mobile terminal (PANU) to neighboring NAPs (NAP A and NAP C), and requests paging to the NAPs at the same time (refer to (3)). Thereafter, the neighboring APs (NAP A and NAP C) serving as a master have the inquiry information of the mobile terminal (PANU), so that they skip the inquiry scan procedures and immediately perform the paging procedure (refer to (5)), and, at this time, the mobile terminal (PANU) as a slave continuously performs the page scan (refer to (4)). Further, if a new connection is completely established between the mobile terminal (PANU) and the AP (NAP C) (refer to (6)), the newly connected AP (NAP C) transfers an acknowledge message INC_ACK(BD_ADDRPANU) to neighboring APs (NAP A and NAP B) through wires (refer to (7)). The neighboring APs that have received the acknowledge message abort attempts to make a connection with the mobile terminal, and then the AP newly connection-established with the mobile terminal creates and configures a new baseband connection through the Link Manager Protocol (LMP) (refer to (8)), and sets up a Blue Network Encapsulation Protocol (BNEP) connection with the Logical Link Control and Adaptation Protocol (L2CAP) (refer to (9)).
FIG. 2 is a view for showing signal flows when a handoff occurs by an NAP in the APR. If a handoff occurs by an NAP, a BNEP handoff request message is initiated at the NAP (refer to (11)) and a disconnection response is performed in the NAP (refer to (12)), which is different from FIG. 1 in which the handoff is triggered by the PANU. The subsequent operations have the same signal flows.
As another example for the conventional handoff processing method, there is the Indirect-Transmission Control Protocol (I-TCP) proposed by A. Barke and B. R. Badrinath (Reference material: “I-TCP: Indirect TCP for Mobile Hosts”, in Proc. Of the 15th IEEE ICDC, May 1995). I-TCP is a handoff processing protocol that completely splits the TCP connection between the Correspondent Host (CH) and the mobile terminal into the wired TCP connection and the wireless TCP connection to protect the wired TCP connection from problems occurring in the wireless TCP connection so that the TCP congestion does not occur.
As yet another example for the conventional handoff processing method, there is the Bluetooth TCP Booster (BTB) proposed by D. Melpignano and D. Siropaes (Reference material: “Bluetooth TCP Booster”, in Proc. Of IEEE VTC, pp. 2167-2141, 2001). The BTB is installed between the AP layer-2 Bluetooth and the layer-3 IP, which is a handoff processing approach that monitors TCP downstreams, estimates an average arrival time of upstream ACK packets, and, if the ACK packet arrival is delayed compared to the estimated time, sends a fake ACK resizing the reception window size to zero to prevent the TCP downstreams transmitted from a Correspondent Host. That is, the BTB estimates the average arrival time of the ACK packets by using a local timer, so that the ACK packets do not arrive within the estimated time if a handoff occurs. At this time, the BTB sends to the Correspondent Host the fake ACK resizing the reception window size to zero instead of a mobile terminal, and the Correspondent Host decides the fake ACK is a normal ACK and prevents the TCP downstreams, so that a data loss does not occur during handoff. However, the data loss cannot be avoided that occurs between the time when a mobile terminal passes a wireless cell boundary and the time when a handoff is triggered.
However, in the above conventional handoff methods, the APR has a problem of poor service quality since it has still the unsolved TCP exponential backoff due to the data loss during handoff which is another problem of the hard handoff. That is, when using the handoff scheme of the APR, it is difficult, under the environments having bad wireless channel characteristics, to set up fast connections that are advantageous to the fast handoff scheme of the APR; thus, the TCP exponential backoff occurs due to downstream data loss during handoff, causing a problem of decreasing the TCP data throughput even though a link is re-established.
Further, the existing TCP has to be modified for the I-TCP, and the I-TCP splits the TCP connection into the wireless connection and the wired connection, so that an AP between the wireless and wired connections has to be provided with all the wireless and wired TCP stacks. Accordingly, the I-TCP slows down its processing speed, which may be a waste with respect to resource utilization. Further, there may exist a bottleneck region between the split wireless and wire connections, which causes a problem of delaying smooth flows of the entire network.
Further, in the BTB, an agent located on the layer 2.5 manages the layer-4 TCP and ACK packets, which causes a problem-violating the layering concept. Moreover, if there occurs a handoff in a state that the data processed by the BTB agent processes and completely transmits some parts of multiple baseband packets consisting of Internet Protocol (IP) packets, the IP packet is received from the beginning in the communications with a new AP, causing a problem of increasing overhead with respect to the Bluetooth. Further, in the current TCP mechanism, the BTB is an approach that transmits ACK packets one by one every time the TCP segments are received one by one rather than an approach transmitting ACK packets in use of a timer, so that it has a problem that an AP can not generate ACK packets for triggering communications due to blocked TCP streams.